A polymer is a large molecule formed by the union or polymerization of smaller molecules called monomers. For example, polymerization of vinyl chloride yields the polymer polyvinyl chloride (hereinafter referred to as "PVC"). A polymer formed by the polymerization of two or more different monomers (also called "comonomers") is known as a copolymer. For example, the copolymer polyvinyl chloride acetate is made by the polymerization vinyl chloride and vinyl acetate.
PVC has been used for a number of years in the manufacture of soft, flexible films for food packaging, in molded rigid products (such as pipes, fibers, upholstery and bristles), and in a variety of other products, including electric wire and cable-coverings, film finishes for textiles, raincoats, belting, gaskets and shoe soles.
The addition of fluorescent or phosphorescent pigments to PVC films has lead to their application as signal tapes. These signal tapes are useful on signs and for designating emergency exits and pathways upon lighting failure in buildings, as well as on public transportation such as airplanes and buses. The character of the absorption and emission of light from these pigmented films is dependent upon the nature of the pigment. The emission of light from a film which contains a fluorescent pigment is very, short-lived. Once the excitation source is removed, fluorescent emission typically ceases within nano- to milliseconds. On the other hand, a film which contains a phosphorescent pigment may continue to emit light from seconds to minutes after the excitation source is extinguished. Phosphorescence or long-lived emission is commonly referred to as "afterglow." PVC films which contain phosphorescent pigments provide long-lived emission of light and overcome the practical limitations associated with the lack of afterglow associated with fluorescent films.
Phosphorescent PVC films are traditionally made by the polymerization of vinyl chloride in the presence of the phosphorescent pigment at relatively high temperature. In addition to the phosphorescent pigment and vinyl chloride, polymer formulations typically contain stabilizers to reduce degradation during the polymerization process. Despite the presence of stabilizers, the harsh manufacturing conditions of such films invariably lead to some decomposition of the components of the formulation. Thus, phosphorescent emission from films prepared in this manner suffer from low intensities and shortened durations (i.e., weak and abbreviated afterglows).
Accordingly, there is a need in the art for phosphorescent PVC films which possess improved phosphorescent properties, including strong emission intensities and long emission durations. In addition, there is a need in the art for an improved formulation for the manufacture of such phosphorescent PVC films, as well as methods relating thereto. The present invention fulfills these needs, and provides further related advantages.